The invention relates to curable biscitraconimide containing compositions, copolymers of biscitraconimides with olefinically unsaturated materials, a process for curing these copolymers, and to articles of manufacture comprising the biscitraconimide(co)polymers.
Biscitraconimides are known compounds and can be prepared by the methods disclosed in, "The Synthesis of Biscitraconimides and Polybiscitraconimides," Galanti, A. V. and Scola, D. A., Journ. of Poly. Sci.: Polymer Chemistry Edition, Vol. 19, pp. 451-475, (1981), the disclosure of which is hereby incorporated by reference. These biscitraconimides are polymerized to tough amber-colored films that exhibit good thermal stability. In addition, the article points out that NMR analysis shows that the observed ratio of methyl protons at 2.1 ppm. to the methylene protons at 1.6 ppm. in the biscitraconimides is lower than the theoretical ratios for the imide monomers. The difference is explained as being due to a small degree of polymerization that could occur when the acid is dehydrated thermally.
"The Synthesis of Bisitaconamic Acids and Isomeric Bisimide Monomers," Galanti, A. V. et al., Journ. Poly. Sci.: Polymer Chemistry Edition, Vol. 20, pp. 233-239 (1982) also discloses a method for the preparation of biscitraconimides in the form of an isomeric mixture of the citraconic and itaconic imides.
In "The Development of Tough Bismaleimide Resins," Stenzenberger, H. D., et al., 31st International SAMPE Symposium, Vol. 31, pp.920-932 (1986) it is disclosed that bismaleimides are prime candidates for carbon fiber reinforced composites because of their properties. However, the article also points out that these materials tend to be brittle. Thus, several attempts have been made to improve the fracture toughness of the bismaleimides. First, the bismaleimides have been cocured with reactive elastomers such as carboxy terminated acrylonitrile-butadiene rubbers. Also, the bismaleimide polymers have been modified with comonomers which copolymerize via a linear chain extension reaction and include both diene type copolymerization reactions and "ene"-type copolymerization reaction. Thirdly, the bismaleimides have been modified with thermoplastic materials. Finally, the bismaleimides have been cured in the presence of ionic curing catalysts such as imidazoles and tertiary amines including diazobicyclo-octane (DABCO). Among the alternatives for improving the fracture toughness of the bismaleimides was the incorporation of diallyl benzenes. As shown in table 3 it was found that these materials provided acceptable properties only up to 40 weight percent of the maleimide. Once more than 40 weight percent of the maleimide was employed, a significant decrease in the flexural strength and flexural modulus was observed.
In "Bismaleimide Resins the Properties and Processing of `Compimide` BMI Resins," Segal, C. L., et al., 17th Nat. SAMPE Conference 17pp. 147-160 (1985) formulated bismaleimides are modified with acrylonitrile-butadiene rubbers to produce an increased fracture toughness. However, it was concluded that the rubber is not compatible with the base resin and an additional, pre-reaction step was necessary to accomplish the modification.
Bismaleimide-styrene compositions are known from European Patent Application 0 108,461 published on May 16, 1984, and, "Molecular Structure and Properties of Bismaleimide Styrene Cross-linked Copolymers," Winter, et al., Proceedings of the 3rd Annual Int. Conf. on Crosslinked Polymers, Lucerne, Switzerland, pp, 291-303, (1989). Example II of the European Patent application also discloses a copolymer of styrene, acrylic acid, bismaleimide and biscitraconimide. However, this material contains a relatively large amount of the bismaleimide and acrylic acid and thus is quite different from the material in accordance with the present invention. Most importantly, the copolymer including a biscitraconimide disclosed in this patent application has a strain % of 0.9, which is significantly lower than compositions of the present invention.
Finally, UK patent application GB 2,010,866 discloses copolymers of styrene with compounds having an N-substituted imide group including bismaleimides and monomaleimides. In this patent it is briefly mentioned that one of the possible imides that can be employed is N,N'-4,4'-diphenylmethane biscitraconimide. However, no examples using this compound are included, and this compound is an alkylated arylene biscitraconimide.
Generally, the bismaleimide resins require difficult processing conditions, exhibit solvent retention in prepregs, have a high melting point and high curing temperatures are required for the monomer. In addition, the maleimide polymers are often brittle due to the high cross-link density obtained in the network polymers. The foregoing body of prior art reflects the need for bisimide resin systems which are easily processable and exhibit improved mechanical and physical properties.